Existing methods, well-known tools, and well-used tool are as follows.
One of them is a file having a concave portion.
When the file is used for chamfering and rounding off an edge of a fret, the top portion of the fret is undesirably scraped, and the height of the fret is unconsciously changed. This ruins a leveling of the frets.
The angle of the file in use is not stable because the angle of the file depends on how the file is grasped with a hand. Therefore, the angle of the polished surface is readily changed, and the resultant final shape is not regular.
The size of the file is not suitable for all kinds of frets, and thus it is required to prepare various sizes of files, which is costly.
One of the other tools is a triangular file.
In order to avoid scratching the fretboard, the triangular file is modified by rounding off edges of a normal triangular file. However, the modified triangular file still has a problem of the angle and the stability, because the file is hand-held even though the file is placed on a portion of the fretboard.
Since the fret is in a long shape and its grip is positioned outer side of the fretboard, it is not easy to hold the file by hand in parallel to the fret. Generally, the triangular file is large and has a rough surface. Therefore, when the triangular file is used, the fret is readily scraped too much, and thus the triangular file is not suitable for precision operation.